STRATIGRAPHIC RANGE AND BIOSTRATIGRAPHIC UTILITY OF THE CRETACEOUS SHARK GENUS PTYCHODUS

Ptychodus is a cosmopolitan Albian to Maastrichian selachian genus. In North America, its principal occurrences are in strata deposited in the Cretaceous Western Interior seaway with some rare records along the Atlantic and Pacific margins. There are eight named species of Ptychodus, with numerous subspecies recognized by some authors. The species of Ptychodus can be divided into two groups based on stratigraphic ranges. The group of P. anonymus, P. decurrens, P. occidentalis, P. polygyrus, P. mammillaris and P. whipplei has a stratigraphic distribution from middle Cenomanian through early Coniacian. Beginning in the late Turonian through the middle of the Coniacian there is a substantial turnover and a reduction in the number of species, and the second group arises, which consists of only two species, P. latissimus and P. mortoni. Biostratigraphically, these taxa are not very useful other than to indicate a prolonged evolutionary turnover between the late Turonian and the middle Coniacian. The species of Ptychodus have the following stratigraphic ranges: (1 ) P. anonymus, middle Cenomanian-late Turonian; (2) P. decurrens, Albian-middle Turonian in North America with reports of Albian-Turonian in Belgium; (3) P. latissimus, earliest Coniacian-early Campanian; (4) P. mammillaris, middle Turonian-Coniacian; (5) P. mortoni, middle Coniacian-Maastrichian; (6) P. occidentalis, middle late Cenomanian-Turonian; (7) P. polygyrus, Turonian in North America, Turonian-Santonian in Europe; (8) P. whipplei, middle Cenomanian-early Coniacian.